Take Athanasius Kircher for example, a Jesuit priest who, in the 17th century, made a brilliant and original study of Egyptian hieroglyphics, which was all the more original because he had no knowledge of the subject and worked entirely through ineffective guesswork. Limited opportunities to study Eastern languages in Europe continued to produce similar if less colourful follies until the middle of the 19th century. The famous Orientalist Richard Burton was some way into his self-taught study of Arabic before he discovered he should be reading the language right to left instead of left to right.

What gives For Lust of Knowing its bite and drive is that throughout Irwin is shadow boxing with the American Arab Edward Said's Orientalism. This famous book published in the 1970s, took the emerging fashion for deconstruction to claim that all Western views of Arabia were a construct, fashioned to give Europe a sense of its own identity compared to an exotic other and then used to justify imperial domination. Said's use of evidence for his claim has long been dismissed by specialists but his views still exert a lot of popular influence.

But then Arabic is a very limited speciality. One of the fascinating things to emerge from the book is how little interest the West has taken in either Arabic or Islam. Most of the scholars Irwin identifies as the true Orientalists were men who seldom left their universities. Many did not speak but only read Arabic, and of those who visited Arab countries, few knew enough colloquial Arabic to speak to Arabs. As far as they were involved in imperialism at all, they usually took the part of the colonised. As enthusiasts, they have sometimes bequeathed us an exaggerated idea of the extent of, say, learning and religious tolerance in medieval Andalusia.

While the stereotypes of Oriental decadence and luxury which Said describes definitely exist, the otherness of the Arab world has often been idealised to put aspects of the West in an unfavourable light. What emerges beyond doubt is that, although there were Orientalists, there has never been Orientalism, but a plethora of often contradictory views, dictated by the eccentric ideas of individuals.

Books which painstakingly disassemble (rather than deconstruct) popular misconceptions are usually dull because detail is inevitably less enjoyable to absorb than myth. This one certainly is not. It must be said, however, that Irwin's polemical vigour sometimes allows him to fall into the same faults he finds in Said.

There are inaccuracies. Irwin writes that Lord Macaulay, the Victorian politician and poet who was sometimes critical of cherished Orientalist positions, never visited India. He lived and worked there for four years. And there are occasions when Irwin allows personal animosity to influence his judgement. In disparaging the 19th-century French writer Ernest Renan, Irwin claims that because Renan worked only from medieval translations from Arabic into Latin for his study of Ibn Rushd (Averroes), Renan's Arabic must have been weak. Renan, however, claimed that he was unable to find as much as a single line of Ibn Rushd in Arabic, either in the libraries of Europe or the Middle East, which seems a plausible claim to my admittedly inexpert mind.

And Irwin remains a shadow boxer, not because Said is now dead, nor because the inaccuracies in Orientalism have been aired many times before, to Said's complete indifference (the unreliability of Orientalism seems to be only thing everybody with any opinion about the Middle East agrees on). What is at issue is not a clash of civilisations but a paradigm shift. Irwin is a champion of philology, the 19th-century study of language, which has fallen away to nothing after the introduction of the discipline of linguistics after the Second World War.

Said was a post-modernist, and the central preoccupation of post-modernism is meaning, which post-modernists see as separate from language, which for them is the hieroglyphics of power. Like Athanasius Kircher they begin with the confidence that they already know the meaning of everything, and organise the facts on the ground with a paranoiac intensity, to reveal the oppressor triumphing over the oppressed yet again.

The generation of Orientalists who taught Irwin Arabic in the 1960s were convinced they were the last in their line. They assumed that the post-colonial Arab states would naturally found departments of Arabic in their own universities, which were bound to be superior to any found in Europe or the United States. This has not happened, for a variety of reasons, not least because the resurgence of fundamentalist Islam does not embrace textual criticism. Some radical Islamicists have embraced post-modernism, finding its criticism of Enlightenment values extremely useful.

In Said's last writings, when he knew he was dying, he called for the return of philology. He thought that its assumption of common humanity, against the anti-humanism of post-modernism, and its belief in disinterested truths discovered through lust of knowing alone, were the only means by which Israelis and Palestinians could find common ground. Too late, alas: the genie is out of the bottle.

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